The primary aim of the Health and Retirement Study (HRS) cooperative agreement is to collect and distribute multi-disciplinary data to support research on aging. Because of their value in assessing health status and elucidating pathways connecting social experience to health, biomarkers have become a key element of the HRS approach to measurement. This revision application aims to build on this foundation and augment the biomarker component of the current HRS renewal cycle by collecting samples of venous blood from participants in the longitudinal study. A selective set of assays will be performed on samples and data released to the research community. The choice of assays is constrained by the amounts of blood to be drawn, the type of tubes to be used in collection, a non-fasting sample, and the processing delays inherent in our field effort. Within these constraints the proposed assays will enhance harmonization with other large population studies of aging and independently advance research on mechanisms of social disparities in health and aging. In addition, our proposed assays on the aging of the immune system and related molecular and cellular age-related changes which are accessible through blood will provide an innovative and potentially important understanding of some of the mechanisms underlying differences in aging for social and demographic subgroups. Although the HRS is not intended to be primarily a bio-repository study, we do recognize the potential for future innovation in laboratory assays and scientific advances and so storage for future research is also an aim of this revision.